1. Field of the Invention
The present invention relates to a solid-state imaging apparatus equipped with an area image sensor used for a household video camera, a digital still camera, a camera for a mobile telephone, etc., and a method for producing the solid-state imaging apparatus.
2. Description of the Related Art
FIG. 13 is a circuit diagram showing a configuration of a conventional solid-state imaging apparatus 90. Photosensitive cells 98 composed of photodiodes 95, transfer gates 96, amplifier transistors 92, and reset transistors 97 are arranged in a matrix (3 row×3 column).
Drains of the amplifier transistor 92 and the reset transistor 97 are connected to a common drain line 306. A source of the amplifier transistor 92 is connected to a vertical signal line 15, as shown in FIG. 13. One end of the vertical signal line 15 is connected to a load transistor 305, and the other end thereof is connected to a noise suppressing circuit 12. Outputs of the noise suppressing circuit 12 are connected to horizontal transistors 14 driven by a horizontal driver circuit 13. Each photosensitive cell 98 is driven by a vertical driver circuit 11.
FIG. 14 is plan view showing a configuration of the photosensitive cells 98 provided in the conventional solid-state imaging apparatus 90. A signal of the photodiode 95 is read to a floating diffusion layer 91 through the transfer gate 96. The signal that has been subjected to voltage conversion in the floating diffusion layer 91 is applied from a floating diffusion layer contact 203 to a gate 304 of the amplifier transistor 92. A source/drain of the amplifier transistor 92 is connected to the common drain line 306 and the vertical signal line 15. A signal charge in the floating diffusion layer 91 is discharged to the common drain line 306 through the reset transistor 97.
FIG. 15 is a cross-sectional view along a plane XYZW shown in FIG. 14. The photodiode 95 composed of an n-type photodiode diffusion layer 402 and a p-type leakage blocking layer 403 is formed in a P-type semiconductor substrate 9.
A gate electrode of a MOS transistor constituting the transfer gate 96, the reset transistor 97, and the amplifier transistor 92 has a double-layered structure of a polysilicon layer 406 and a salicide layer 407.
The floating diffusion layer 91 has the salicide layer 407 on a double diffusion layer composed of an LDD diffusion layer 404 and a source/drain diffusion layer 405.
A source/drain of the MOS transistor has the salicide layer 407 on the double diffusion layer composed of the LDD diffusion layer 404 and the source/drain diffusion layer 405. The salicide layer 407 does not transmit light, so that it is removed from an upper portion of the photodiode 95.
FIGS. 16 to 19 are cross-sectional views showing a method for producing the conventional solid-state imaging apparatus 90. As shown in FIG. 16, after a device separating layer 502 is formed on a semiconductor substrate 9, a resist 501 is formed in a predetermined pattern by photoetching, and an n-type photodiode diffusion layer 402 and a p-type leakage blocking layer 403 are formed by ion implanting.
After the resist 501 is removed, a polysilicon layer 406 to be gate electrodes of MOS transistors constituting the transfer gate 96, the reset transistor 97, and the amplifier transistor 92 is formed, as shown in FIG. 17. Thereafter, a salicide blocking film 503 is formed so as to cover the photodiode 95, and then, a LDD diffusion layer 404 is formed so as to be self-aligned with the polysilicon layer 406 by ion implanting.
Then, as shown in FIG. 18, an LDD oxide film 504 is deposited so as to cover the salicide blocking film 503, the polysilicon film 406, and the LDD diffusion layer 404. Then, as shown in FIG. 19, the LDD oxide film 504 is removed by anisotropic etching, whereby parts of the LDD oxide film 504 remain on both sides of the polysilicon layer 406 deposited thick in a vertical direction. A source/drain diffusion layer 405 is formed so as to be self-aligned with the LDD oxide film 504. Thereafter, metal materials such as titanium (Ti), cobalt (Co), etc. are deposited by sputtering, followed by heating. As a result, only the portions where the semiconductor substrate and polysilicon are exposed are salicided, and a salicide layer 407 remains.
In the above-mentioned configuration of the photosensitive cells in the conventional solid-state imaging apparatus, the floating diffusion layer 91 temporarily accumulates a signal of the photodiode 95. At this time, when there is a pn-junction opposite-direction leakage current in the floating diffusion layer 91, the leakage current is superimposed on the signal to generate noise.
The time for signal charge to remain is shorter than that for the photodiode 95 to subject incident light to photoelectric exchange and store it. Therefore, a requirement for a pn-junction opposite-direction leakage current is not so strict as in a photodiode; however, when the floating diffusion layer 91 is produced in the same way as in source/drain of other transistors, a pn-junction opposite-direction leakage current is increased and causes serious noise. When this noise is large, the sensitivity of the solid-state imaging apparatus is decreased to degrade an S/N ratio of a signal, etc.